The wetting of the surfaces of optical elements with liquids, in particular water, generally has adverse effects on their optical properties. For example, wetting may cause contaminating substances such as salts to form on their surfaces. This a relevant issue in particular in immersion lithography, in which an immersion fluid is introduced between a last optical element of a projection lens and a light-sensitive substrate.
It is known that the wetting of an optical element outside an optically used region or diameter at which the optical element is immersed into the immersion fluid can have adverse effects on its optical properties: If the wetting liquid evaporates on a surface outside the optically used surface region, cold of evaporation is produced. Therefore, a heat sink that has an adverse effect on the thermal equilibrium of the optical element may form in the wetted region. For example, in the case of optical elements in the form of lenses, the refractive index of the material used as the main body may change locally as a result, which can lead to imaging errors.
In order to avoid wetting of optical elements with water or to remove water quickly from them, it is known to provide optical elements with hydrophobic coatings. For example, WO 2008/031576 A1 describes an optical element of a material that is transparent to wavelengths in the UV range, to which a hydrophobic coating has been applied. The hydrophobic coating is formed outside the optically free diameter, or outside the optically used region, of the optical element and has at least one layer that is UV-resistant and preferably absorbent to UV radiation at a wavelength of less than 260 nm. The optical element may be formed as a plano-convex lens, on the planar surface of which there is formed a conically shaped lens part. The hydrophobic coating may be formed on the conical lateral surface of the conically shaped lens part and/or on the plane surface.
It has been found that the liquid-repellent properties of a hydrophobic coating may decrease during the service life of the optical element (generally several years), for example as a result of contaminants deposited there and/or due to the long-term exposure to UV radiation.